There has been a proposal to mount a heater unit on a steering wheel rim in order to warm driver's hands in cold weather. The patent documents 1 and 2 listed below, by way of example, disclose a heater unit for use in a steering wheel which is composed of a substrate and a cord-shaped heater arranged on the substrate in a predetermined pattern. The patent documents disclose various material such as foamed resin sheet, foamed rubber sheet, rubber sheet, non-woven fabric, woven fabric or the like as the substrate.